.world instance != Lemmy (lemny)
The instance has some rules for content, and discussions around celebrating a murder are getting caught up in that.
There might be some overstepping of moderation in places, but it must be a bit of a minefield for the mods.
The best thing for you to get the experience you want from Lemmy is to engage in those discussions in an instance that suits your moderation preferences.
Just like on Reddit, that depends on the moderators of the sub/com. It really couldn't work any other way. .world seems to often err on the side of being inoffensive for the sake of broader appeal. (And probably also not getting in hot water with their hosting partners, acquaintances, etc.)